lab 6 - Liquid-Liquid Base Extraction of Benzoic Acid from...

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Liquid-Liquid Base Extraction of Benzoic Acid from Acetanilide followed by Recrystallization/Product Identification by Melting Point Determination Mayank Kumar March 23, 2007 Methods and Background The purpose of this experiment was to separate a 1:1 mixture of benzoic acid and acetanilide through extraction with aqueous sodium hydroxide and purify the products by recrystallization. Liquid-liquid extraction is used to remove an organic compound from a mixture, and it involves the partitioning of compounds between two immiscible phases (phase distribution), which is governed by selective dissolution. In this technique, a solute, A , is dispersed between S x , the extracting phase, and S 0 , the original phase; typically the immiscible liquids used are water and an organic solvent, such as dichloromethane. In liquid-liquid extraction, the two immiscible liquids essentially compete for solute A , and once equilibrium has been established, the ratio of concentrations of A in each phase will remain constant. K , the partition coefficient, expresses the amount of A , in g/mL, in each phase, and it is approximated by dividing the solubility of A in the extracting solvent S x by the solubility of A in the original solvent S 0 ; if K is greater than 1, A will be mostly in S x . Furthermore, the proportion of A remaining in S 0 after multiple extractions F A is equal to the volume of original solvent V 0 divided by the quantity of K multiplied by the volume of extracting solvent V x plus V 0 , with the whole expression raised to n , the number of extractions performed. From this equation, it can be deduced that multiple extractions become significant as the value of K decreases to increase the quantity of solute extracted. When choosing a suitable extracting solvent, some requirements must be kept in mind: it must not be chemically reactive with the components of the mixture, it must be immiscible with the original solution, it must have a higher K for the desired component being extracted, and it should be separable from the solute. Carboxylic acids contain functional groups that are polar (due to interatomic electronegativity differences), and unless they contain fewer than about six carbon atoms, they will typically be insoluble or only slightly soluble in water ( K is less than 1) due to the hydrophobic properties of the portion of the molecule containing carbon (R or Ar); however, carboxylic acids are soluble in organic solvents such as dichloromethane, which have moderate polarity, so if they are extracted with water, they will generally remain in the organic solvent phase. The amino functional group is also polar and hydrophilic, and amines possessing more than six carbon atoms are insoluble or
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only slightly soluble in water ( K is less than 1) because of the hydrophobic properties of the R or Ar groups. If a carboxylic acid is extracted with a basic aqueous solution, it will be converted into the
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lab 6 - Liquid-Liquid Base Extraction of Benzoic Acid from...

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